The invention is concerned with vehicles having wheels or tracks, an engine and a transmission. The purpose of a transmission is to vary the speed of the shafts driving the wheels or axles in order to vary the speed of the vehicle. For most vehicles speed can be adjusted in one of two ways: either the engine throttle can be adjusted to increase or decrease the speed of the engine, or the ratio of the transmission can be changed. In some applications, for example agriculture, forestry, and construction it is preferable to be able to set a desired forward speed without changing the engine speed (for example when a forage harvester is driven from a tractor's power take off, the engine must rotate at a substantially fixed speed for the forage harvester to operate properly), and it is for this reason that such vehicles are either provided with transmissions having a large number of ratios, or transmission where the drive ratio is continuously variable. Vehicles not having such requirements, such as a car need only be provided with a transmission having relatively few ratios, for example four or five, because for such vehicles it is acceptable for some of the variation in forward speed to be provided by variation in the vehicle's engine speed.
In addition to being used in applications where it is necessary to vary forward speed without varying engine speed, vehicles such as agricultural, forestry and construction vehicles are also used in applications where varying the forward speed by a combination of varying the engine speed and the transmission ratio is acceptable. For instance, agricultural tractors are often used on the road in haulage applications, e.g. for hauling loads from fields to stores.
When used in haulage applications the driver of an agricultural tractor must change the engine speed and the transmission ratio. In a continuously variable transmission he can select from an infinite number of ratios. In a transmission having discrete ratios, he may have to select a ratio from a choice of 32 possible ratios. This makes acceleration and deceleration a tiring task as the driver must make so many changes to the transmission ratio and the engine speed. Furthermore, when the desired travelling speed has been reached the driver may have to make further adjustments to the gear ratio due to changing loads on the vehicle, for example if a hill is encountered the load on the vehicle will rise and the engine speed will fall. If the torque reserve of the engine is accounted for by the increase in load it will be necessary for the driver to change the transmission ratio.
It would therefore be desirable to automate the operation of the transmission and throttle in order to optimise the efficiency of operation of the vehicle, and to reduce driver fatigue.